Evaluating a geometric or material property of a multilayered structure

ABSTRACT

A structure having a number of traces passing through a region is evaluated by using a beam of electromagnetic radiation to illuminate the region, and generating an electrical signal that indicates an attribute of a portion (also called “reflected portion”) of the beam reflected from the region. The just-described acts of “illuminating” and “generating” are repeated in another region, followed by a comparison of the generated signals to identify variation of a property between the two regions. Such measurements can identify variations in material properties (or dimensions) between different regions in a single semiconductor wafer of the type used in fabrication of integrated circuit dice, or even between multiple such wafers. In one embodiment, the traces are each substantially parallel to and adjacent to the other, and the beam has wavelength greater than or equal to a pitch between at least two of the traces. In one implementation the beam is polarized, and can be used in several ways, including, e.g., orienting the beam so that the beam is polarized in a direction parallel to, perpendicular to, or at 45° to the traces. Energy polarized parallel to the traces is reflected by the traces, whereas energy polarized perpendicular to the traces passes between the traces and is reflected from underneath the traces. Measurements of the reflected light provide an indication of changes in properties of a wafer during a fabrication process.

CROSS-REFERENCE TO PRIORITY APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/521,232 filed Mar. 8, 2000, which is incorporated by reference hereinin its entirety.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is related to and incorporates by reference herein intheir entirety, the following commonly owned, copending U.S. patentapplications:

Ser. No., 09/095,805, entitled “AN APPARATUS AND METHOD FOR MEASURING APROPERTY OF A LAYER IN A MULTILAYERED STRUCTURE,” filed Jun. 10, 1998,by Peter G. Borden et al;

Ser. No. 09/095,804, entitled “APPARATUS AND METHOD FOR EVALUATING AWAFER OF SEMICONDUCTOR MATERIAL,” filed Jun. 10, 1998, by Peter G.Borden et al; and

Ser. No. 09/274,821, entitled “APPARATUS AND METHOD FOR DETERMINING THEACTIVE DOPANT PROFILE IN A SEMICONDUCTOR WAFER,” filed Mar. 22, 1999, byPeter G. Borden et al.

BACKGROUND

In the processing of a semiconductor wafer to form integrated circuits,a number of traces are normally formed over an underlying layer. Thetraces are normally used to interconnect transistors and other devicesin the integrated circuits. Such traces may have widths under 0.2micrometers (microns), pitches (center to center spacing) under 0.4microns and aspect ratios exceeding 4:1.

Depending on the stage of the processing, it may be necessary to measureproperties of various portions of a wafer, such as the properties of thetraces and/or the properties of the underlying layer. However, thepresence of traces can interfere with conventional measurements thatexamine open areas (areas not covered by traces).

SUMMARY

A structure having a number of lines supported by a layer in contactwith the lines (also called “multi-layered structure”) is evaluated inaccordance with the invention by illuminating a region (also called“illuminated region”) containing several lines, using a beam ofelectromagnetic radiation, and generating an electrical signal (e.g., byuse of a photosensitive element) that indicates an attribute (e.g.,intensity or optical phase) of a portion (also called “reflectedportion”) of the beam reflected from the region. As more than one line(and therefore more than one portion of the layer in contact with thelines) is being illuminated, the reflected portion and the electricalsignal generated therefrom do not resolve individual features in theilluminated region, and instead indicate an average measure of aproperty of such features. In contrast, most prior art methods measure aproperty of an individual feature in such a multi-layered structure. Thejust-described lines can be either conductive (in which case they arealso referred to as “traces” or non-conductive, depending on theembodiment.

In one embodiment, the acts of “illuminating” and “generating” arerepeated in another region (of the same structure or of a differentstructure) also having multiple traces. The electrical signals beinggenerated from light reflected by different regions can be automaticallycompared to one another to identify variation of an average property(e.g., average thickness of the layer in contact with the traces, oraverage resistance per unit length of the traces) between the regions.Instead of (or in addition to) the just-described comparison, the valuesof such a signal can be plotted in a graph to indicate a profile of asurface in the region. A value being plotted can be an absolute value ofthe reflected portion alone, or can be a value relative to anotherportion that is reflected by another surface in the same region (whichindicates the average distance therebetween), or by the same surface inanother region (which indicates an average profile of the surface).

Such measurements can identify variations in properties in asemiconductor wafer of the type used in fabrication of integratedcircuit dice, or between multiple such wafers (e.g., values measuredfrom a reference wafer and a production wafer or between two successiveproduction wafers can be compared). Identification of a change in aproperty between two or more wafers is useful e.g., when performing suchmeasurements during wafer fabrication, so that process parameters usedto fabricate a next wafer (e.g., creating the above-described layer orthe traces) can be changed as necessary (in a feedback loop), togenerate wafers having material properties within acceptable limits.Note, however, that structures other than semiconductor wafers (e.g.,photomasks that include a glass substrate and are used to form thewafers, or an active matrix liquid crystal display) can also beevaluated as described herein.

In a first example, there is a transmissive medium (such as air) locatedbetween a source of the beam (also called probe beam) and theilluminated region. In one implementation, another beam (also called“heating beam”) is used in addition to the probe beam, to modulate thetemperature of the traces (e.g., at a predetermined frequency).Reflectance of the lines changes with the change in temperature. Thereflected portion (which depends on reflectance), and hence thegenerated signal also oscillates (e.g., as the predetermined frequency).Such an oscillating signal is measured by e.g., a lock-in amplifier, andthe measurement is repeated in another region. If all lines in theilluminated region are conductive (also referred to as “traces”),comparison of measurements from different regions (e.g., which may be inthe same location in different die of a wafer, or which may be in thesame die in different wafers) indicates a change in the averageresistance per unit length (and therefore the corresponding change incross-sectional area) between traces in the respective regions (ifconductivity is constant).

A series of measurements from regions adjacent to one another (or evenoverlapping one another) in the longitudinal direction of the traces,when plotted in a graph along the y axis with the x axis indicatingdistance along the longitudinal direction yields a profile of the traces(which may be used to detect, e.g. global nonuniformity such as a dimpleor a dome). Depending on the specific variant, the probe beam and theheating beam can each be coincident with or offset from the other.

In another implementation, multiple traces in a region of a structure ofthe first example are each substantially parallel to and adjacent to theother, and the beam has wavelength greater than (or equal) to a pitchbetween two adjacent traces. In one such embodiment, the probe beam ispolarized (e.g., by a polarizing optical element interposed between asource of the beam and the structure), although a nonpolarized probebeam can be used in other embodiments. A polarized probe beam can beused in several ways, including, e.g., orienting the probe beam so thatthe electrical field vector for the electromagnetic radiation is at apredetermined angle relative to the traces.

When the probe beam is polarized perpendicular to the traces, the tracesdo not reflect the probe beam. Instead, the probe beam passes betweenthe traces and is reflected from underneath the traces, e.g. by chargecarriers of a semiconductor layer, or by a surface of an oxide layer, orboth. Such light which is reflected from underneath the traces can beused to identify variation in a property of features underneath thetraces (averaged over the features that are illuminated). The portionreflected by charge carriers is relatively small (e.g., 1/10⁴ or less)as compared to the portion reflected by an underlying surface, andtherefore has a negligible effect on an overall measurement of a steadysignal (also called “DC” component). If necessary, the portion reflectedby charge carriers can be measured by modulating the number of chargecarriers and using a lock-in amplifier to measure the portion of areflected light that is modulated (also called “AC” component) asdescribed elsewhere herein. The charge carriers can be created by a beamhaving an oscillating intensity (or oscillating phase). In this variant,the reflected portion has an intensity (or phase) that is modulated inphase with modulation of the charge carriers (and can be measured by useof a lock-in amplifier).

When the probe beam is polarized parallel to the longitudinal directionof the traces, the above-described reflected portion (that is used togenerate the electrical signal) is reflected by the traces. Thereflected portion can be used to identify variation in a property thatis averaged over the traces. A probe beam polarized parallel to thetraces can be used with a heating beam that is also polarized parallelto the traces, and in such a case effectively on the traces interactwith the heating beam, and are heated more, as compared to heating by anunpolarized heating beam. Alternatively, the just-described probe beam(also called “parallel polarized beam”) can be used with another probebeam that is polarized perpendicular to the traces (also called“perpendicular polarized beam”). The two polarized beams can begenerated from the same beam, e.g., by a polarizer or a polarizingoptical element (such as a Wollaston beam splitter), or by a combinationof such optical elements (e.g. Wollaston beam splitter followed by apolarizer). A polarizer here refers to any optical element or set ofoptical elements whose output is a beam with a single direction ofpolarization.

In one embodiment, a portion of the parallel polarized beam reflected bythe traces, and a portion of the perpendicular polarized beam reflectedfrom underneath the traces interfere, and the interference pattern isused to generate an electrical signal. As noted above, the electricalsignal indicates a profile of the underneath surface when the beams areoffset. When the parallel polarized beam and the perpendicular polarizedbeam are coincident, the electrical signal indicates a distance betweenthe underneath surface and a surface of the traces exposed to thetransmissive medium (also called “exposed surface”). Note that theexposed surface of the structure can be formed by a surface of thetraces and a surface of the layer that interdigitates between the traces(the layer surface and the trace surface can be substantiallyco-planar—within the same plane or in planes that are separated fromeach other by less than 10% of the width of the traces) and suchsurfaces can be formed, e.g., by chemical mechanical polishing.

The two probe beams that are polarized mutually perpendicular to eachother can each be oriented at 45° relative to the traces, so that atleast a portion of each beam is reflected from the exposed surface ofthe structure. In such a case, the electrical signal obtained from thetwo or more reflected portions indicates a profile of the exposedsurface, assuming the two beams are offset from one another, and thesurface containing the traces has a constant profile. An optionalpolarizing beam splitter can be used to limit the measurement to the twoportions that are reflected by the traces (or to the two portions thatare reflected by a surface underneath the traces when profiling theunderneath surface). Therefore, illuminating a region containing two ormore traces allows use of the wafer as a polarizer to measure an averageproperty of features underneath the traces that are otherwiseinaccessible.

In a second example, the traces are separated from the transmissivemedium by a layer (also called “exposed layer”) included in thestructure. One method used with the second example measures a signalobtained from interference between a portion of the probe beam reflectedby the traces, and another portion reflected by a layer formed over thetraces. Reflection of a perpendicular polarized beam by the exposedlayer overcomes a prior art problem of illuminating a region containingtraces, because the traces do not adversely affect the perpendicularlypolarized light (e.g., the traces reflect parallel polarized light). Thejust-described method does not require a heating beam. This method alsohas the advantage of being able to measure a property of traces buriedunderneath the exposed layer.

In a variant of the just-described method, both portions are reflectedby the traces, and each portion is offset from the other thereby toyield a signal indicative of a profile of the surface of traces(although the traces are located underneath the exposed layer). In sucha method, the to-be-reflected portions of a probe beam can be polarizedmutually perpendicular to each other and oriented at 45° relative to thetraces. Instead of mutually perpendicularly polarized beams, two beamsthat are polarized parallel to one another and also parallel to thetraces also can be used, e.g., to obtain a surface profile of the traces(that are located underneath the exposed layer).

Furthermore, instead of being offset from one another, the parallelpolarized beams can be coincident, with one beam being the probe beamand the other beam being the heating beam. In such a case, the measuredsignal provides an indication of a property of the traces, although thetraces are located underneath the exposed layer. If the two beams thatare polarized parallel to one another (e.g., a probe beam and a heatingbeam) are both oriented perpendicular to the traces (a first set)underneath the exposed layer, a property of a second set of traceslocated underneath the first set can be determined. Furthermore, insteadof a heating beam, a pump beam can be used to generate charge carriersin a layer located underneath the traces.

One implementation combines two of the above-described methods, by usingtwo beams that are respectively polarized parallel and perpendicularrelative to the longitudinal direction of a set of traces in thestructure. In this implementation, two electrical signals for twomeasurements in the two polarization directions are generatedcontemporaneously (e.g., just before, during or just after each other).Simultaneous generation of the two electrical signals provides anadvantage in speed, as compared to sequential generation of the twosignals. Such electrical signals can provide measures of properties ofboth traces and a layer underneath the traces, so that a wafer can beaccepted or rejected in a signal operation.

In another embodiment, the probe beam is nonpolarized (or has circularor elliptical polarization so that both orthogonal polarizationcomponents are simultaneously present in the single probe beam). In oneimplementation of this embodiment, the method includes generating asingle electrical signal from the portion of light reflected when anonpolarized (or circular or elliptical polarized) probe beam is used.In another implementation, the method includes contemporaneousgeneration of two electrical signals based on measurement of twocomponents of the reflected portion: a first component that is polarizedin a direction perpendicular to the traces, and a second component thatis polarized in a direction parallel to the traces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates, in a perspective view, a portion of a structurehaving a number of traces in a region illuminated by a probe beam inaccordance with the invention.

FIG. 1B illustrates, in an elevation view in the direction 1B of FIG.1A, the relationship between the diameter Dp of the probe beam and thepitch p between the traces.

FIG. 1C illustrates, in a flow chart, acts (including illumination ofmultiple traces illustrated in FIGS. 1A and 1B) being performed duringwafer fabrication, in one embodiment of the invention.

FIG. 1D illustrates, in a block diagram, one embodiment of a measurementapparatus of this invention being used with various devices thatfabricate the structure of FIG. 1A.

FIGS. 1E-1J illustrate, in partial cross-sectional views, asemiconductor wafer at various stages of fabrication in the apparatus ofFIG. 1D.

FIG. 1K illustrates, in a plan view of the arrangement illustrated inFIG. 1A, the relation between the electric field vector (of a probe beamthat is linearly polarized) and the traces.

FIG. 2A illustrates, in a partial cross-sectional view, the relationshipbetween two polarized components of a probe beam and the light reflectedby or passing between the traces in a semiconductor wafer.

FIG. 2B illustrates, in a cross-sectional view, a structure havinggrooves 208A-208M that contain a gas, such as air that acts as a numberof non-conductive traces, and which can also be used as a polarizer asdescribed herein.

FIG. 2C illustrates, in a graph, relation between pitch (on the x axis)and extinction ratio (on the y axis) for light of a fixed wavelength,wherein the extinction ratio is a ratio of light (intensity) transmittedto a region underneath the traces (FIG. 2A) in the perpendicular andparallel directions.

FIGS. 2D, 2E and 2G illustrate, in block diagrams, alternativeembodiments that use one or more polarized components of a probe beam.

FIG. 2F illustrates, in a graph, the change in reflectance of thestructure of FIG. 2B as a function of depth Gd of the grooves.

FIGS. 3A and 3B illustrate, in graphs, a change in reflectance of theregion illuminated by the probe beam (illustrated in FIG. 1A) as afunction of thickness of an insulative layer located underneath thetraces (plotted along the x axis in μm) for FIG. 3A or thickness oftraces for FIG. 3B.

FIGS. 3C-3E illustrate, in graphs, relationships between measurements ofthe two polarized components in one example.

FIG. 4A illustrates, in a flow chart, acts performed during waferfabrication in one implementation of the embodiment illustrated in FIG.1C.

FIG. 4B illustrates variations in measurements across a wafer in oneexample of a uniformity map obtained towards the end of the processillustrated in FIG. 4A.

FIGS. 5A and 5B illustrate, in partial cross-sectional views,measurement of properties of a semiconductor wafer having an exposedlayer formed on traces in two variants.

FIGS. 6A and 6B illustrate, in cross-sectional views, two mutuallyperpendicular components of a probe beam that are offset from oneanother for use in obtaining a surface profile.

FIGS. 6C and 6D illustrate, in plan views, the orientation ofpolarization direction of the beams of FIGS. 6A and 6B respectivelyrelative to the traces.

FIG. 6E illustrates use of a polarizing beam splitter 1520 to generatethe two components of the probe beam illustrated in FIGS. 6C and 6D.

FIGS. 7A-7E are similar or identical to the corresponding FIGS. 6A-6Eexcept that the polarization directions of the two beams are parallel toone another.

FIG. 8 illustrates, in a partial cross-sectional view, the variousdefects in a semiconductor wafer that can be identified by use of themeasurements described herein.

FIG. 9 illustrates, in a high-level block diagram, a circuit included inmeasurement apparatus 125 of FIG. 1D in one embodiment.

DETAILED DESCRIPTION

A structure 10 (FIG. 1A) is multilayered, and contains a number of lines11A-11N (A ≦I≦J≦N; N being the total number of lines) passing through aregion 11 (also called illuminated region) of a layer 13. Lines 11A-11Nhave an index of refraction different from the index of refraction oflayer 13, and therefore reflect a probe beam that is directed at region11. Note that lines 11A-11N need not be conductive, although. in oneembodiment the lines are conductive. For this reason, in the followingdescription, the term “lines” is used generically, and when referringspecifically to embodiments involving lines that are formed ofconductive material, the term “traces” is used. Embodiments involvingother kinds of lines are apparent to the skilled artisan in view of thedisclosure (e.g. see the description below in reference to FIGS. 2B and2F). Structure 10 can be (but is not required to be) a wafer of the typecommonly used to manufacture integrated circuit dies. Note also thatlines 11A-11N need not be parallel to each other (except when apolarized probe beam is used as discussed below in which case lines11A-11N are at least substantially parallel to each other).

Structure 10 is evaluated in one embodiment of the invention by focusing(see act 22 in FIG. 1C) a beam 12 (FIG. 1A) of electromagnetic radiationon region 11 (which is defined to be the entire region illuminated bybeam 12 on an exposed surface 13S of structure 10). Beam 12 has adiameter d (at surface 13S of structure 10) that is selected to beseveral times larger than the width w of a line 11I For example,diameter d can be 2 microns and width w can be 0.15 microns (so thatseven lines are simultaneously covered by beam 12).

Note that beam 12 merely illuminates region 11 and may or may not befocused on surface 13S (which is a surface of structure 10 exposed to atransmissive medium 15 such as air). In different embodiments, beam 12is focused on (a) surface 13S, (b) surface 14S, (c) between surfaces 13Sand 14S, (d) surface 16, or (3) above surface 13S. Therefore, beam 12does not resolve individual features in region 11 (unlike a scanningmicroscope of the prior art which can resolve the individual features).Instead, beam 12 is used to obtain an average measure of one or moreproperties in illuminated in region 11, e.g., of lines 11A-11N, or oflayer 13 or a combination thereof, or some other material or feature inregion 11. layer 13 or a combination thereof, or some other material orfeature in region 11.

A portion of beam 12 is reflected by region 11, and is used to generate(e.g., as illustrated by act 23 in FIG. 1C) an electrical signal (e.g.,by use of a photosensitive element) that indicates an attribute (e.g.,intensity or optical phase) of the reflected portion. The measuredattribute in turn is used as an average measure of a property of amaterial in region 11. For example, if the just-described acts 22 and 23are performed in one region 11, a stage that supports structure 10 movesstructure 10 so that a different region is illuminated, and then theseacts 22 and 23 are repeated. Therefore, this embodiment involvesstepwise movement (“hopping”) from one region to another region ofstructure 10 when performing measurements of the type described herein(as opposed to a scanning microscope of the prior art that continuouslymoves (“sweeps”) a beam of electromagnetic radiation relative to astructure). In the hopping process, the stage holds structure 10stationary for a moment (e.g., 1 second) while a measurement is taken inone region, and then moves to another region (e.g., of the samestructure).

Two regions in which measurements are made can be separated from eachother, e.g., by distance which is same as the diameter Dp of beam 12.Alternatively, the two regions can touch each other or even overlap eachother. When overlapping one another, the centers of the two regions maybe separated by a small fraction of the diameter, e.g., by ( 1/10) Dp orless. Regardless of how close the regions are, the hopping processyields discrete values (one for each region) as compared to the sweepingprocess which yields a continuous signal. As described elsewhere herein,the regions can be physically located in different structures, so thatan alternative embodiment involves hopping from structure to structure(when hopping among regions). A combination of the just-described twotypes of hopping can also be used (i.e., moving between regions of thesame structure and also moving between regions of different structures).

Note that the just-described “hopping” can be performed from one regionto a next region that touch each other, and a measurement from eachregion can be plotted in a graph, e.g., to indicate a profile of asurface across the regions. As described elsewhere herein, suchmeasurements provide an average profile (in view of nonresolution of theindividual features in the illuminated region). In another embodiment,the hopping is performed between regions that overlap one anotherthereby to provide a more realistic measure of the average profileacross these regions, as compared to non-overlapping regions.

The electrical signals obtained by the measurements are optionallycompared (e.g., in act 24) either against each other or against apredetermined limit, to identify a change in a property (such as thethickness of layer 13 or thickness lines 11A-11N) between the regions.An electrical signal indicative of reflectance changes in response to achange in a property of features (such as layer 13 or lines 11A-11N)containing the material being evaluated in structure 10. Note that theelectrical signal by itself provides an average measure of the propertyin the region due to the region having a size that is larger (e.g., byan order of magnitude) than to size of an individual feature.

Note also that only changes that cause a property to fall outside apredetermined range are flagged in one embodiment. Such a predeterminedrange can be same as the limits beyond which a structure is rejected asbeing unacceptable (or can be smaller than such limits to allow acorrection to be made even before an unacceptable structure isfabricated). Note that the property being measured can be an averagedimension of the features in region 11 (such as thickness of traces) oran average material property of such features (such as the averageresistance per unit length of the traces).

Comparisons of such signals from different regions (of a structure orstructures) may be performed manually, although in other examples suchcomparisons are performed automatically (by a programmed computer).Alternatively, the electrical signals generated in act 23 can be plottedto obtain a two dimensional image of structure 10 as a whole, so thatthe image indicates changes in property (also called “materialproperty”) between such regions. Instead of a two-dimensional image, theelectrical signals can be plotted in a graph along the y axis, with thex axis representing regions in structure 10.

As noted above, the just-described regions can be inside a singlestructure 10, or spread across multiple such structures (e.g., in areference structure that has known material properties, and a productionstructure that is currently being fabricated and whose properties areyet to be determined, or even multiple production structures).Identification of changes in a property between two or more structuresis useful e.g., when performance of such measurements is interleavedbetween fabrication processes, so that one or more process parametersused to fabricate a next structure (such as creating traces or a layeradjacent to the traces, as illustrated by optional act 21) can bechanged as necessary (e.g., as illustrated by optional act 25) tofabricate structures having properties within acceptable limits.

Note that acts 21, 24 and 25 described above in reference to FIG. 1C areoptional, and may or may not be performed, depending on the embodiment.For example, the generated electrical signals may be manually evaluated.Alternatively, such evaluations (manual or automatic) may be performedindependent of the fabrication processes of the structures. In oneembodiment, the above-described structure 10 is implemented as asemiconductor substrate (also called “wafer”) of the type used infabrication of integrated circuit dice. In this embodiment, a processingunit 100 (FIG. 1D) creates integrated circuit (abbreviated as “IC”) diceby processing a semiconductor substrate 102 (FIGS. 1D and 1E) to formvarious substrates 103-107 (FIGS. 1F-1J) at intermediate stages in thefabrication of the dies.

In one example, a patterning apparatus 120 deposits a photoresist layer102A on top of an insulation layer 102B which in turn is formed onsilicon substrate 102C (FIG. 1E). In this example, an etching apparatus121 exposes and develops photoresist layer 102A to form therein grooves103A-103M (M being the total number of grooves), thereby to form wafer103 (FIG. 1F). Thereafter, an etching apparatus 121 etches through thepatterned photoresist layer 102A to form grooves 104A-104M in insulationlayer 102B, thereby to form wafer 104 (FIG. 1G).

Next, resist layer 102A is removed, and a liner deposition apparatus 122forms a barrier layer 105A (e.g., of tantalum nitride to preventdiffusion of to-be-applied conductive material, such as copper, intoinsulation layer 102B) in etched grooves 104A-104M of insulation layer102B, as illustrated by wafer 105 in FIG. 1H. Then a conductive material106A is blanket deposited on wafer 105, by a deposition apparatus 123thereby to form substrate 106 (FIG. 1I). Note that barrier layer 105A,although present in substrate 106, is not shown for clarity in FIG. 1I.The deposited layer 106A has a thickness t (FIG. 1I) of, for example,1-2μm. Next a polishing apparatus 124 is used to polish back layer 106A(e.g., by 1-2μm), leaving conductive lines 107A-107M (also called“traces”) in grooves of insulation layer 102B, as in the case of adamascene structure.

Processing unit 100 includes a measuring apparatus 125 (FIG. 1D) thatperforms the process described above in reference to FIGS. 1A-1C.Therefore, at any point during wafer fabrication of the type describedabove in reference to FIGS. 1D-1J, a wafer can be subjected to themeasurement process e.g., as illustrated by arrows 110-114. If ameasured signal falls outside a predetermined limit (e.g., exceeds amaximum or falls below a minimum), the fabrication process can beadjusted in real time, thereby to produce more wafers that areacceptable (than if measurements were done after wafers are fabricated).The predetermined limit can be selected after calibration, e.g. from asignal obtained when the measurement process is performed on a waferhaving known properties (which are properties determined by use of anyprior art method).

One embodiment of apparatus 125 includes an optional programmed computer126 that supplies a process parameter (used in the fabrication process)on a bus 115 that is coupled to each of apparatuses 120-124 describedabove. A change in the process parameter can be determined automaticallyby software in programmed computer 126 (e.g. by performing a table lookup), or can be entered by a human operator. Note that in one embodimenta single measurement operation on wafer 107 measures properties oftraces 11A-11N (FIG. 1A) and also of layer 13, so that multiplemeasurement operations are not required.

In one embodiment, traces 11A-11N (FIG. 1A) are each substantiallyparallel to and adjacent to the other (e.g., centerlines CI and CJ oftraces 11I and 11J that are adjacent to each other form an angle of lessthan 25° relative to one another). In this embodiment, beam 12 isselected to have a wavelength greater than or equal to pitch p betweentwo adjacent traces 11I and 11J. In one implementation of such anembodiment, measurement apparatus 125 (FIG. 1D) determines, between twoor more acts of fabricating substrate 102, an average measure of thethickness t of layer 13 in region 11 (FIG. 1A), simply by measuring theintensity of the portion of beam 12 reflected from region 11.

Note that another beam (also called “pump beam”), in addition to beam12, can be used to create charge carriers in layer 14 (e.g., asdescribed in the related patent application, U.S. patent applicationSer. No. 09/095,804) if layer 14 is formed of a semiconductor material.In one such example, beam 12 (also called “probe beam”) contains photonshaving energy lower than or approximately (within 10%) equal to thebandgap energy of a semiconductor material in layer 14. In the example,charge carriers may be modulated in any one or more of several ways: (1)by photogeneration of additional charge carriers; (2) movement ofbackground carriers due to change in voltage potential caused byillumination or some other way.

In one embodiment the charge carriers are modulated at a frequency thatis sufficiently low to avoid creation of a wave of charge carriers. Ifso modulated, the reflected portion of beam 12 is also modulated at thejust-described frequency, in phase with modulation of the chargecarriers (and the reflected portion of beam 12 can be measured by use ofa lock-in amplifier as stated in the just-described patent application).

Beam 12 can be linearly polarized, circularly polarized, ellipticallypolarized, nonpolarized or some combination thereof, depending on theimplementation. So, in one implementation, probe beam 12 isnonpolarized, and one embodiment generates a single electrical signalfrom the reflected portion. Such an electrical signal (as a whole)provides an average measure of the thickness t of a layer 13 (FIG. 1B)that supports traces 11A-11N. In one embodiment, in addition to thenonpolarized probe beam 12, an additional beam such as the heating beamdescribed in the related patent application Ser. No. 09/095,805 is used(as described below) to illuminate multiple traces.

In this embodiment, a modulated component of the electrical signal (asmeasured by a lock-in amplifier) provides a measure of a property (suchas thickness) of traces 11A-11N. The modulated component of theelectrical signal, obtained from measuring the change in reflectance oftraces 11A-11N, is sufficiently small relative to the overall electricalsignal (due to reflectance of nonpolarized probe beam 12 by region 11)so that the overall electrical signal can be used as a measure of aproperty of layer 13.

Therefore, a measure of the modulated component and of the overallelectrical signal (or its steady component) identify a change inproperties of different layers of a structure, and such measurements canbe performed in a single operation. Instead of using a heating beam, ifa pump beam of the type described in the related patent application Ser.No. 09/095,804 is used, then a change in a property of a semiconductorlayer 14 (FIG. 1A) can be identified by use of a lock-in amplifier tomeasure the modulated component.

Instead of nonpolarized beam, a circularly or elliptically polarizedbeam can also be used as described herein for a nonpolarized beam(except that separate calibration is required for an ellipticallypolarized beam; specifically, in the case of elliptically polarizedlight, the intensities in the two directions are different: for example,if the ratio of intensity in the two directions is 2:1(parallel:perpendicular), then the parallel signal will be twice asstrong for the same reflectivity, and reflection in the paralleldirection must be divided by 2 to compare to the reflection in theperpendicular direction).

In one embodiment, a reflected portion of a nonpolarized probe beam 12is passed through a polarizer or a polarizing beam splitter to generateone or both components that have orthogonal polarization directions. Forexample, a polarizer may be used to select an individual polarizationdirection that may be oriented parallel (or perpendicular depending onthe orientation of the polarizer) to a set of traces, and so that thephotocell detects only parallel polarized light (and the perpendicularpolarized light is blocked). Alternately, a polarizing beam splitterseparates the unpolarized light into two orthogonal polarizationcomponents, for instance, aligned parallel and perpendicular to a set oftraces. The parallel and perpendicular polarized components are thenintercepted by separate photodetectors to simultaneously measure theirindividual intensities.

In the above-described embodiment, probe beam 12 can be either polarizedor nonpolarized. In one implementation, probe beam 12 is linearlypolarized even prior to being incident on structure 10 (e.g., by apolarizing element interposed between a source of beam 12 and structure10). The polarizing element can be a polarizing beam splitter availablefrom Melles Griot of Irvine Calif. (see, for example part number 033 PBB012). A polarized probe beam 12 can be used in several ways, including,e.g., orienting beam 12 (FIG. 1K) so that the electric field vector vthereof is at a predetermined angle θ (such as 0°, 90° or 45°) relativeto the longitudinal direction 1B (FIG. 1A) of traces 11A-11N.

In one example, a probe beam 203 i (FIG. 2A) is polarized perpendicular(i.e., θ=90°) to traces 211A-211N, which appear transparent to beam 203i due to the orientation, as long as the wavelength exceeds the pitch.Therefore, probe-beam 203 i has energy in the form of beam 203 t thatpasses through layer 202 (that is at least partially transmissive), andthe remaining energy of probe beam 203 i is reflected (e.g., by surface202 s) in the form of reflected portion 203 r or absorbed. Thetransmitted portion 203 t passes between traces 211A-211N in thedirection of incidence DI, because traces 211A-211N act as a polarizer,as described in, e.g., the Optics Handbook, pages 10-72 to 10-77. Asdescribed therein, the transmittances T₁ and T₂ for the grid of traces211A-211N are:

$\begin{matrix}{\left( T_{1} \right)_{\bot} = \frac{4\; n\; A^{2}}{1 + {\left( {1 + n} \right)^{2}A^{2}}}} & (1) \\{\left( T_{1} \right)_{} = \frac{4\;{nB}^{2}}{1 + {\left( {1 + n} \right)^{2}B^{2}}}} & (2)\end{matrix}$where n=refractive index of (transparent) substrate material

-   (T₁)_(⊥)=transmittance for radiation polarized perpendicular to the    traces.-   (T₂)_(∥)—transmittance for radiation polarized parallel to the    traces.    The general expressions for A and B are:

$\begin{matrix}{\frac{1}{A} = {\frac{4d}{\lambda}\left\{ {{\ln\left\lbrack {\csc\frac{\pi\left( {d - a} \right)}{2d}} \right\rbrack} + \frac{Q_{2}{\cos^{4}\left\lbrack {{{\pi\left( {d - a} \right)}/2}d} \right\rbrack}}{1 + {Q_{2}{\sin^{4}\left\lbrack {{{\pi\left( {d - a} \right)}/2}d} \right\rbrack}}} + \mspace{50mu}{\frac{1}{16}{\left( \frac{d}{\lambda} \right)^{2}\left\lbrack {1 - {3\;\sin^{2}\frac{\pi\left( {d - a} \right)}{2d}}} \right\rbrack}^{2}\cos^{4}\frac{\pi\left( {d - a} \right)}{2d}}} \right.}} & (3) \\{{B = {\frac{4d}{\lambda}\left\lbrack {{\ln\left( {\csc\frac{\pi\; a}{2d}} \right)} + \frac{Q_{2}{\cos^{4}\left( {\pi\;{a/2}d} \right)}}{1 + {Q_{2}\sin\;{c^{4}\left( {\pi\;{a/2}d} \right)}}} +} \right\rbrack}}\mspace{45mu}{\frac{1}{16}{\left( \frac{d}{\lambda} \right)^{2}\left\lbrack {1 - {3\;\sin^{2}\frac{\pi\; a}{2d}}} \right\rbrack}^{2}\cos^{4}\frac{\pi\; a}{2d}}} & (4) \\{{{where}\mspace{14mu} Q} = {\frac{1}{\left\lbrack {1 - \left( {d/\lambda} \right)^{2}} \right\rbrack^{\frac{1}{2}}} - 1}} & (5)\end{matrix}$These relations hold for traces 211A-211N having trace width a andspacing d, assuming λ>2d. Eqs. (3) and (4) are in error by less than 1percent when λ>2d; for the condition λ>d, the error is less than 5percent but increases for still shorter wavelengths.

When the trace width a is equal to the width of the spaces between thetraces (d−2a), Eqs. (3) and (4) are simplified since 3−a=a−d/2:

$\begin{matrix}{B = {\frac{d}{\lambda}\left\lbrack {0.3466 + \frac{0.25Q_{2}}{1 + {0.25Q_{2}}} + {0.003906\left( \frac{d}{\lambda} \right)^{2}}} \right\rbrack}} & (6) \\{A = {{1/4}B}} & (7)\end{matrix}$

Note that although traces 211A-211N are described for one embodiment, analternative embodiment is for lines that are not conductive.

Another structure 207 (FIG. 2B) has grooves 208A-208M that hold air orother gas. Grooves 208A-208M are formed in an insulative layer 208 (thatis supported on a substrate 209) by etching, e.g. as described above inreference to FIG. 1F. When a probe beam 203 i is incident on structure207, grooves 208A-208M act in a manner similar to that described herein(above and below) in reference to traces 211A-211N, except for anydiscussion related to a heating beam.

Specifically, there is a difference in the index of refraction betweenthe air in grooves 208A-208M and in layer 208, and the optical effect issimilar to the effect when there are traces in the grooves.Specifically, probe beam portion 203 t reflected from structure 207 hasan intensity that is dependent on the average depth Gd (FIG. 2F) ofgrooves 208A-208M (FIG. 2B). In the example illustrated in FIG. 2F,curves 301 and 303 are formed by measurements from illuminating thetraces with a perpendicularly polarized beam in a structure having a 1.0μm thick insulative layer, and having a trace width of 0.18 μm. Curve301 is obtained when the electric field (also referred to as “TE”) isoriented along grooves 208A-208M, whereas curve 302 is obtained with theelectric field oriented perpendicular to grooves 208A-208M (i.e.magnetic field is oriented along the grooves). Curves 302 and 304 areformed by similar measurements from use of a parallel polarized beam,when the trace width is 0.13 μm.

The intensity of reflected portion 302 r is also a function ofpolarization direction and of the thickness tg between the bottomsurface of grooves 208A-208M and substrate 209. Performing a reflectancemeasurement with one probe beam and then repeating the measurement witha probe beam of a different wavelength, yields two measurements that areused with charts to extract the depth Gd and thickness tg, e.g., asdescribed below in reference to FIG. 3A (i.e., to resolve an ambiguity).Alternatively, such reflectance measurements can be made by use ofeither one of two probe beams that have mutually perpendicularpolarizations. Of course, both can be used successively (i.e., one afteranother) to obtain two reflectance measurements that can be used (withcharts) to look up each of depth Gd and thickness tg. Note that grooves208A-208M in layer 208 do not act as a sheet of metal or conductor whenilluminated by a beam polarized parallel to the longitudinal directionof the grooves.

For a given wavelength λ of probe beam 12, as the pitch p is reducedbelow λ(and the number of traces in region 11 is increasedcorrespondingly) a parallel polarized beam is reflected more effectivelyand a perpendicular polarized beam is transmitted more effectively.Specifically, an extinction ratio increases with reduction of pitch, asillustrated by curves 221-223 in FIG. 2C which illustrate the extinctionratio for light at a wavelength of 0.98 μm as a function of the pitch inmicrons. The extinction ratio is the ratio determined by dividing thetransmission for light polarized perpendicular to the lines by thetransmission for light polarized parallel to the traces. Curve 222 isfor trace width equal to half the pitch. Curve 221 (solid line) is forthe case of traces being 10 % wider than the half-pitch (trace widthequals pitch/1.8 μm). Curve 223 (dashed line) is for the case of traces10 % narrower than the half-pitch (trace width equals pitch/1.2 μm).

Therefore when pitch is approximately equal to the wavelength, bothpolarized components are transmitted equally. So, in one embodiment,wavelength greater than pitch is used to yield a large extinction ratio(e.g., greater than 2). When pitch is greater than or equal to 0.85 μm,the probe beam diameter Dp becomes on the order of the width of thetraces, so that eventually there is no transmission of the incidentlight, and instead there is full reflection.

Thereafter, in one embodiment beam 203 t is reflected (thereby to formreflected portion 203 r) by a surface (not shown in FIG. 2A) betweenlayer 202 and an underlying layer. Reflected portion 203 r passes backbetween traces 211A-211N in the direction DR that is opposite to theincidence direction DI, and is measured. Perpendicular polarized beam203 i is used in one implementation with an additional pump beam asdescribed in the related patent application, U.S. patent applicationSer. No. 09/095,804 to generate charge carriers and a lock-inamplifier-provides a measure of a property of a layer underneath traces211-211N. In another implementation, reflected portion 203 r is useddirectly (without any additional beam) as an average measure of thethickness of layer 13 across multiple such regions (e.g., in a singlewafer).

In another example, a probe beam 204 i (FIG. 2A) is polarized parallelto traces 211A-211N, and both beams 203 i and 204 i are coincident (onthe same region 211) and both illuminate the traces 211A-211N. Amajority of the energy of beam 204 i is reflected by traces 211A-211N,as reflected portion 204 r. Reflected portion 204 r can be used todetermine a property of traces 211A-211N, e.g., by measuring reflectancedirectly or by measuring interference between the two reflected portions203 r and 204 r. Note that a change in reflectance can indicate acorresponding change in grain structure due to change in surfaceroughness. Roughness can also be measured by measuring light scattering(e.g., as indicated by a ratio of intensity of diffuse and specularreflection). Alternatively, instead of the probe beam 203 i, a heatingbeam can be used as described in the related patent application, U.S.patent application Ser. No. 09/095,805. Specifically, the heating beamhas a power (also called “heating power”) modulated at a frequency whichis selected to be sufficiently small to cause a majority of the heat totransfer by diffusion from region 211. In one example, the heating beamhas a wavelength of 0.83 microns, has an average power of 10 milliwatts,a diameter of 2 microns and is modulated at 2000 Hertz.

The modulation frequency of the heating beam is selected to besufficiently small to ensure that at any time the temperature of traces211A-211N is approximately equal to (e.g., within 90% of) thetemperature of these same traces 211A-211N when heated by an unmodulatedbeam (i.e., a beam having constant power, equal to the instantaneouspower of the modulated beam). For example, the modulation can besinusoidal between 0 and 50 milliwatts, i.e., P=50 sin (2π ft), where fis the modulation frequency. In such an example, at the time when themodulated power has an instantaneous value of 25 mW, the temperatureunder the heating beam approximately equals (e.g., is no less than 90%of) the temperature obtained with a heating beam having constant power,e.g., 25 mW.

In one embodiment, the modulation frequency is selected to cause alltraces 211A-211N illuminated by the heating beam to be at substantiallythe same temperature relative to one another (e.g., varying less than10% between adjacent traces). Such a linear response condition occurswhen the thermal wavelength λ (which is the wavelength of a thermal wavethat is formed in the structure) is at least an order of magnitudelarger than the diameter Dp of the illuminated region 11.

Therefore, when a heating beam is modulated, the temperature T (andtherefore the reflectance) of traces 211A-211M is also modulated inphase with modulation of the heating beam (under linear responseconditions). Reflected portion 204 r (which is sensed to generate anelectrical signal) is also modulated, in phase with modulation of theheating beam. The modulated electrical signal is detected by use of alock-in amplifier as stated in the patent application Ser. No.098/095,805. The modulated electrical signal can be used to identifyvariations in one or more material of the traces (e.g., resistance perunit length which is indicative of cross-sectional area). Note that aheating beam and a probe beam can be offset from one another, forexample by a distance of 5-8 μm because the effect of the heating beam(the linear thermal response) is noticeable for a greater distance(e.g., 10-15 μm) before reaching room temperature.

In one embodiment, an apparatus 800A (FIG. 2D) is used to practice oneor more methods described herein. Specifically, apparatus 800A includesa lens 802 that collimates a beam generated by a laser 801, thereby toprovide a beam 803. Depending on the implementation, laser 802 cangenerate a beam that is not polarized, or polarized in a direction 45°relative to the traces. Apparatus 800A includes two polarizers 804 a and804 b (such as polarizing beam splitting cubes). Polarizer 804 a islocated in the path of beam 803, and transmits light polarized in theplane of the paper and deflects light polarized perpendicular to theplane of the paper. Thus beam 820 transmitted by polarizer 804 a ispolarized in the plane of the paper. A beam 821 that is deflected bypolarizer 804 a is sent to an absorber (for safe disposal).

Instead of using polarizer 804 a, apparatus 800A can be configured touse another polarizer 804 b that is located offset from the path of beam803. Polarizer 804 b transmits light polarized perpendicular to theplane of the paper and deflects light in the plane of the paper. Beam820 is incident on a beam splitter 805 that is also included inapparatus 800 a. Beam splitter 805 passes a portion (e.g., 50%) of theincident light as beam 822 which is focused on structure 807 by anobjective lens 806 located therebetween. Light reflected from structure807 is deflected by a beam splitter 805 to form beam 808 which isincident on a detector 809.

In one implementation, the parts described in the following table areused to form apparatus 800A.

-   -   801 980 nm laser diode model SDLO-2597-160-BN (Spectra Diode        Labs)    -   802 Collimating lens Thor Labs P/NF230FC-B followed by 3×        anamorphic prism, Melles Griot P/N 06GPA004.    -   804 a,b Cube beamsplitter, Part 05FC16PB.7, available from        Newport, Irvine, Calif.    -   805 Cube beamsplitter, Newport P/N 05BC17MB.1    -   806 100X objective lens, Part 1-LM5951 available from Olympus,        Tokyo, Japan.    -   809 Si PIN photodiode. Hamamatsu S2386-8K    -   810 Same as 804    -   812 a,b Same as 809

Hamamatsu is in Hamamatsu, Japan. Newport is in Irvine, Calif. Thor Labsis in Newton, N.J. Spectra Diode Labs is in San Jose, Calif.

In addition a Wollaston prism may be inserted (described later). This ismade by Karl Lambrecht, part number MWQ12-2.5am-V810. A polarizer may beplaced following the Wollaston prism, such as a Polarcor™ LinearPolarizer. Newport P/N05P309AR.16.

Note that polarizers 804 a and 804 b can be replaced by a half-waveplate located in the path of beam 803 (i.e., inline between lens 803 andbeam splitter 805). In this case, laser 801 is polarized. The half-waveplate rotates polarization of beam 803 by 90°, thereby to cause beam 810to have orthogonal polarization. The half-wave plate performs rotationof polarization to provide a probe beam having polarization in eitherdirection without loss of power (whereas when a beam splitting cube isused, 50% of the power is deflected and lost).

In another embodiment, apparatus 800B (FIG. 2E) includes a laser 801that generates a beam 803 that is either unpolarized or circularlypolarized (thereby containing components in both polarizationdirections). As described above, beam splitter 805 passes only a portionof beam 803 as beam 830, while another portion 808 is incident on apolarizer 810 (which can be, e.g., a polarizing beam splitter). Notethat beam 808 is a return beam from laser 801. When beam 808 isunpolarized or circularly polarized, beam 808 contains components inboth polarization directions.

Polarizer 810 passes one polarization component to detector 812 a asbeam 811 a, and deflects the other polarization component as beam 811 bto detector 812 b. Detectors 812 a and 812 b simultaneously providemeasurements of the individual intensities of beams 811 a and 811 b(which represent the parallel and perpendicular polarization directionsdepending on the orientation of the pattern on the wafer 807 withrespect to the orientation of beam splitter 810). Note that a polarizingbeam splitter can be included in the apparatus of FIGS. 2D and 2E,between splitter 805 and lens 806. In this case, beam 822 (FIG. 2D) orbeam 830 (FIG. 2E) is split into two components having mutuallyperpendicular polarization directions. As described below, lens 806focuses the two spatially separated beams on wafer 807 (see FIGS. 6C and6D). In addition, a polarizer can be included between prism 805 and lens806. If such a polarizer has its transmission direction oriented at 45°relative to the two polarization directions, then the two spatiallyseparate beams have the same polarization direction (as illustrated inFIGS. 7C and 7D).

One embodiment uses two probe beams that are polarized in the paralleland perpendicular directions relative to traces 211A-211M. In thisembodiment, two electrical signals for the two polarization directionsare used contemporaneously (e.g., just before, during or just after eachother). Depending on the implementation, the two probe beams canoriginate from a single beam that is either nonpolarized (with thecomponents being obtained after reflection, by passage through apolarizing beam splitter), or is polarized at 45° relative to traces211A-211M. Alternatively, the two probe beams can originate in twoindependently generated beams that are polarized in the parallel andperpendicular directions relative to traces 211A-211M.

An electrical signal is obtained by measuring the reflected light whenusing the above-described two probe beams for a production wafer. Such asignal is used (in this implementation) with charts (which may be intabular form or graphical form and which are formed by use of wafershaving known properties) to look up the average thickness of layer 202and the average thickness of traces 211A-211N. For example, when beam 12(FIG. LA) has one of wavelengths 1.48 μm or 0.83 μm, one of respectivelines 326 and 327 (FIG. JA) is used to look up average thickness “t” oflayer 202 (e.g. in the 0.8-1.2 μm range).

To resolve any ambiguity in a reading from the chart, the measurementcan be repeated with probe beam(s) of different wavelengths(s). Forexample, if when using the 1.48 μm wavelength beam (see line 326 in FIG.3A), the reflectance signal is 0.2 (ratio of reflected power to incidentpower; if the incident power is 1 mW and the conversion efficiency is 1V/mW, then the signal is 0.2 volts) then the thickness can be either0.885 μm or 1.105 μm. In the example, if when using the 0.83 μmwavelength beam (see line 327 in FIG. 3A), the reflectance signal is 0.3(ratio of reflected power to incident power, or 0.3 volts with anincident power of 1 mW and conversion efficiency of 1 V/mW) then thethickness is 1.1 μm.

When a beam of electromagnetic radiation falls onto a structure having anumber of layers of different materials, multiple reflections take placewithin the structure. If the distances between the various boundariesare sufficiently small (e.g. less than the ½ the coherence length of thebeam) the reflected beams are coherent with one another, and willinterfere. Several equations presented below describe properties of sucha structure in terms of the reflectance measurements, and can be used toprogram a computer (as would be apparent to the skilled artisan) todisplay the properties or change in properties. The structure mayconsist of l layers. The structure's properties include not only therefractive indexes n_(i) and the thicknesses t_(i) of the layers butalso the refractive indexes n_(i) and n_(m) of the substrate and thetransmission medium.

The angle of incidence θ and the wavelength λ and plane of polarization(⊥ or ∥) of the incident radiations are the external to the structure. Amethod of calculating the transmittance T and the reflectance R of amultilayered structure from the above-described properties is based on amatrix formulation of the boundary conditions of the surfaces (derivedfrom Maxwell's equations). Specifically, it can be shown that the ithlayer can be represented by the following 2×2 matrix

$\begin{matrix}{M_{j} = \begin{bmatrix}{\cos\left( \delta_{j} \right)} & {\frac{i}{u_{j}}{\sin\left( \delta_{j} \right)}} \\{{iu}_{j}{\cos\left( \delta_{j} \right)}} & {\cos\left( \delta_{j} \right)}\end{bmatrix}} & (8) \\{{{where}\mspace{14mu}\delta_{j}} = {\frac{2\pi}{\lambda}\left( {n_{j}t_{j}{\cos\left( \phi_{j} \right)}} \right)}} & (9)\end{matrix}$the quantity n_(j)t_(j) cos φ_(j) often being called the effectiveoptical thickness of the layer for an angle of refraction φ_(j) andwhere u_(j), the effective refractive index, is given by

$\begin{matrix}{u_{j} = \left\{ \begin{matrix}\frac{n_{j}}{\cos\left( \phi_{j} \right)} \\{n_{j}{\cos\left( \phi_{j} \right)}}\end{matrix} \right.} & (10)\end{matrix}$depending on whether the incident radiation is polarized parallel (topcase of u_(j) in equation 10) or perpendicular (bottom case of u_(j) inequation 10) to the plane of incidence.

The angle φ_(j) is related to the angle of incidence θ by Snell's lawn_(m) sin θ=n_(j) sin φ_(j)   (11)

The complete multilayered structure is represented by the product matrixM,M=M₁M₂ . . . M_(j) . . . M_(i−1)M_(i)   (12)

$\begin{matrix}{M = \begin{bmatrix}m_{11} & {im}_{12} \\{im}_{21} & m_{22}\end{bmatrix}} & (13)\end{matrix}$

In the above equations (8)-(13) the refractive index of any absorbingmaterial in the structure must be replaced by its complex refractiveindex ń, defined byń=n−ik   (14)Where k is the extinction coefficient of the material. Even though allthe elements of the layer matrix for such a material are now complex,its determinant will still be unity.

In terms of the elements of the product matrix, the amplitudetransmittance and reflectance coefficients t and r are given by

$\begin{matrix}{t = \frac{2n_{m}}{\left( {X + W} \right) + {i\left( {Y + V} \right)}}} & (15) \\{r = \frac{\left( {X - W} \right) + {i\left( {Y - V} \right)}}{\left( {X + W} \right) + {i\left( {Y + V} \right)}}} & (16)\end{matrix}$whereX=n _(m) m ₁₁ +n _(m) k _(s) m ₁₂,Y=n _(m)n_(s)m₁₂,W=n_(s)m₂₂,V=m ₂₁ −k _(s) m ₂₂,   (17)where m_(ij) are the elements of the matrix in equation 8, n_(s)−ik_(s)is the complex refractive index of the substrate, and nm is therefractive index of the incident medium, which is usually air, so thatn_(m)=1.

The intensity transmittance and reflectance coefficients areT=n _(s) /n _(m) |t| ²   (18)R=|r| ²   (19)

The absorption of a multilayer is calculated fromA=1−T−R   (20)

Note that lines 326 and 327 (FIG. 3A) are for use when a single beam 12(FIG. 1A) is incident on region 11 and includes light polarizedperpendicular as well as parallel to traces 11A-11N (FIG. 1A). Eachpolarization direction of light reflected from region 11 may include twocomponents (reflected by each of surfaces 13 s and 14 s) that interfere.Two such measurements of reflectance using probe beams of the twowavelengths when used with lines 326 and 327 determine a unique value ofthickness of layer 13. Note that pitch “p” is less than half thesmallest wavelength, i.e., p<0.5 μm. Note that if such parallel andperpendicular polarized probe beams are offset from one another, asurface profile is obtained from the reflected signal as discussedbelow.

In a similar manner, another line 328 (FIG. 3B) is used to look upaverage thickness “m” of traces 211A-211N (FIG. 2A) in the illuminatedregion. In the example illustrated in FIG. 3B, a reflectance signal isplotted on the y axis and thickness of traces (in the form of traces ina semiconductor substrate) is plotted along the x axis. In this example,pitch p is 0.36 μm and trace width is 0.18 μm.

Note that the thickness of traces 211A-211N can also be determined froma measure of resistance per unit length (e.g., as described in Ser. No.09/095,05), which indicates cross-sectional area if the line width andconductivity are both substantially fixed (e.g., change less than 10% inthe illuminated region).

In this embodiment, the measurements are repeated at multiple dice in aproduction wafer, and the thickness values are plotted for the variousdice as illustrated in FIGS. 3C-3E. For example, a graph with thethickness of layer 202 plotted along the x axis, and the thickness oftraces 211A-211N plotted along they axis results in points that fallalong a straight line. For example, lines 151 and 152 (FIG. 3C) arefitted to the points plotted for 0.18 μm and 0.25 μm dice respectively.

In the just described example, 0.18 μm and 0.25 μm are half-pitches(e.g., same as trace width, with trace width equal to the space betweenthe traces) so that pitch is 0.36 and 0.5 μm respectively. Note that themeasurements illustrated in FIG. 3C are obtained by use of a heatingbeam to measure thickness m of traces 211A-211N, and by use ofinterference to measure thickness t of layer 202 as described herein.Note, however that points 153 and 154 for die 5 are at a significantdistance from lines 151 and 152 thereby to indicate a problem with thewidth of traces 211A-211M in die 5.

Instead of fitting the thickness measurements to a line, the thicknessmeasurements can be plotted along the y axis, with the x axisrepresenting the various dice as illustrated in FIG. 3D. Note that therelationship between the two lines 155 and 156 reverses for each of dice5, 9, 11 and 12, thereby to indicate a problem with line width in thesedice. Note that the difference between the two thickness measurementscan also be plotted as illustrated in FIG. 3E. A difference in excess ofa control limit (e.g., 0.50) indicates a problem.

Therefore, in one embodiment, the various graphs in FIGS. 3A-3E indicatea problem if there is a discontinuity therein. Instead of, or inaddition to comparing the thicknesses relative to one another, eachthickness can be compared to a range of acceptable thicknesses. Whenthickness falls outside the range, there is a problem (e.g., over orunder polishing or metal deposition problem). Simultaneous generation ofthe two electrical signals for the two thicknesses provides an advantagein speed, as compared to sequential generation of the two signals.

Instead of determining the absolute value of thicknesses “t” and “m” asdescribed above, the measurements can be directly plotted (or comparedto a range) to identify variation in the measurements. When thevariation exceeds a predetermined limit, appropriate acts are performedto correct the situation (e.g., by changing a process parameter used tocontrol fabrication of another wafer). Therefore, reflectance need notbe computed, and instead a signal indicative of intensity of a reflectedportion is used directly.

In one embodiment, a method 400 (FIG. 4A) uses two signals of the typedescribed above to evaluate a wafer as follows. Specifically, in act310, a wafer is inserted into a wafer aligner of apparatus 125, andtraces formed therein are oriented in a predetermined direction relativeto a stage. Next, in act 301, one or more polarized beams having a knownorientation relative to the predetermined direction (i.e., relative tothe traces) are generated, and illuminate the traces. Thereafter, in act302, a property of the array of traces is measured, using a beampolarized parallel to the traces. Computer 126 checks if the measuredproperty is within a predetermined range, and if not a process parameteris adjusted (e.g., via bus 115 described above) as illustrated by act302 a.

Then, in act 303, a property of the layer in which the array of tracesis embedded is measured, using a beam polarized perpendicular to thetraces. Computer 126 checks (in act 303) if the measured property iswithin a predetermined range, and if not another process parameter (oreven the same process parameter described above) is adjusted, asillustrated by act 303 a. Next, computer 126 compares (in act 304) thetwo measurements to one another, and if there is a large deviation yetanother process parameter (or even the same parameter) is adjusted, asillustrated by act 304 a. Then the above-described acts are repeated (inact 305) at a number of sites, to obtain a uniformity map of the typeillustrated in FIGS. 3C-3E.

Note that the measurements can also be displayed to an operation (bycomputer 126) in a two-dimensional map of the wafer as illustrated inFIG. 4B. For example, dies 5,9, 11 and 12 may be shown highlighted(e.g., brightened, darkened or different color or hatched) to indicatemeasurements beyond a control limit (see FIG. 3E). There may bedifferent types of highlighting (e.g., die 5 v/s dies 9, 11, 12) to showthe degree of variation beyond the control limit. Instead of using acontrol limit, all measurements may be displayed (in correspondinglyvarying shades of gray or color).

Such two-dimensional maps indicate variations across the productionwafer (e.g., dies 5, 9, 11 and 12) are located at the periphery of thewafer and therefore indicate a problem at the periphery. An example ofsuch a problem could occur due to voids forming in the metal traces,typically if dies all around the periphery fall outside of a controllimit (in the example dies 13-26 may fall with the control limit and soa different problem may be present). If several dies of a particularregion (e.g., dies 5, 9, 11 and 12 in the bottom right corner) areaffected, there may be a problem in that region, such as a number ofvoids in one or more of the traces in the illuminated region.

If the uniformity is not within control limits, a process parameter isadjusted, as illustrated by act 305 a. If a production wafer passes allthe tests, one or more additional layers are formed on the wafer by thevarious apparatuses 120-124 (FIG. 1D), and then the wafer is returned tothe aligner (in act 310), and the measurement and control acts 301-305are repeated. Note that while forming the additional layers, acts301-305 and 310 can be performed on a different production wafer.

In several of the above-described embodiments, there is a transmissivemedium directly in contact with the traces, between the traces (on thestructure) and a source of the beam. However, in another embodiment, asubstrate 500 (FIG. 5A) has a transmissive layer (e.g., oxide) 501formed over a network of traces 201A-201N, and layer 501 is evaluated bymeasurement apparatus 125 (FIG. 1D). One or more properties of layer 501are measured by use of a probe beam polarized in a direction parallel totraces 201A-201N, a portion of the probe beam being reflected fromunderneath layer 501, by traces 201A-201N.

In one variant of the just-described example, a heating beam 504 is usedin addition to the above-described probe beam. Heating beam 504 can bepolarized parallel to the traces, for heating the traces as describedabove. Alternatively, heating beam 504 can be polarized perpendicular totraces 201A-201N, and therefore passes through layer 507 twice, once inthe incidence direction DI, and a second time in the opposite directionDR after reflection by surface 508 s between layers 507 and 508. So beam504 which covers multiple traces 201A-201N provides an increased heatingeffect (as compared to a heating beam that is incident only on one trace2011 or incident polarized parallel to traces 201A-201N).

Note that the just-described increased heating effect depends on severalfactors such as thickness of traces 201A-201N, the direction ofpolarization relative to the longitudinal direction of traces 201A-201N,and the thickness of layer 507 (which affects the reflected power fromthe underlying structure). In another embodiment, the heating beam ispolarized parallel to the longitudinal direction of the traces. In thiscase, the heating is independent of the thickness of the traces or ofthe thickness of layer 507. This provides heating independent of otherparameters of the structure e.g., if measuring resistance per unitlength.

Therefore, the extinction ratio (FIG. 2C) changes with change inthickness of traces 201A-201N. If the width of traces 201A-201N isfixed, the extinction ratio can be used as a measure of the tracethickness. If trace thickness is fixed, then the extinction ratioprovides a measure of variation in thickness of layer 507. In oneimplementation, a measurement (using a probe beam and heating beam 504)is made immediately after formation of layer 501, and provides a moreimmediate feedback to control the operation of the apparatus (FIG. 1D)used to form layer 501.

Note that depending on the embodiment, layer 501 (FIG. 5A) can be incontact with just one surface of each of traces 201A-201N, oralternatively a layer 502 (FIG. 5B) can be in contact with multiplesurfaces (e.g., three surfaces) of each of traces 201A-201N (e.g.,traces 201A-201N are embedded in layer 502). In one embodiment, twoprobe beams 504A and 504B (FIG. 5B) that are polarized in mutuallyperpendicular directions (e.g., obtained by use of a Wollaston prism)are used to evaluate a structure 509 having traces 503A-503N that areembedded within a layer 502. In this embodiment, beam 504A that ispolarized parallel to traces 503A-503N is reflected as beam SOSA bythese traces, and interferes with a beam 505B which is a portion ofperpendicular polarized beam 504B reflected by surface 502S (assumingthat layer 506 is a substrate that absorbs light). The two reflectedbeams 505A and 505B interfere, and an electrical signal derivedtherefrom provides a profile of traces 503A-503N.

In another embodiment, two beams polarized perpendicular to each otherare used with structure 1501 that has traces 1504A-1504M exposed to thetransmissive medium. In this embodiment, a portion of the parallelpolarized beam 1502 as (FIG. 6A) is reflected by traces 1504A-1504M, anda portion of the perpendicular polarized beam 1502 ap (FIG. 6C) isreflected from a surface 1503S of layer 1503 underneath traces1504A-1504M. Note that traces 1504A-1504M are coplanar as illustrated inFIG. 6A, while the similar traces 1509A-1509M are substantially coplanaras illustrated in FIG. 6B (e.g., the “substantially coplanar” traces maybe embedded in the same layer 1508 to form a single exposed surface1509L).

In the preceding sentence, the terms “coplanar” and “substantiallycoplanar” are used to mean the following. In the case of coplanar, thesurface serves as a reference to measure the profile of the underlyingsurface. Hence, the planarity of lines 1504A-M should be less than 10%of the profile of surface 1503S. Substantially coplanar refers to theprofile of surface 1509L, which could be defined as an order ofmagnitude less planar than underlying reference surface 1508S.

The reflected portions are used to generate an electrical signal whichindicates a profile of the underneath surface 1503S when the beams 1502as and 1502 ap are offset by a distance Do. Offset distance Do may be,for example same as a diameter Dp of probe beam at the illuminatedregions. When the two polarized beams are coincident on the same region(e.g. see region 11 illustrated in FIG. 1A), such an electrical signalindicates a thickness t of the exposed layer(e.g. layer 1503 in FIG.6A).

Note that in structure 1501, exposed surface 1501T of a trace 1504M isco-planar with a surface 1501 L of a layer that interdigitates betweenthe traces 1504A-1504M, and these two surfaces can be formed, e.g., bychemical mechanical polishing. The two probe beams 1502 as and 1502 apcan be generated in any manner, by use of a polarizing optical elementsuch as a Wollaston beam splitting prism and an objective lens focusingthe beams on the structure, e.g., as illustrated in FIG. 6E. Note thatan exposed surface (formed by surfaces 1501T and 1501L) is being used asa reference surface by the parallel polarized beam (and conversely thesurface 1503S can be used as a reference surface by a perpendicularlypolarized beam).

Two probe beams 1502 bs and 1502 bp that are polarized mutuallyperpendicular to each other can each be oriented at 45° relative to thelongitudinal direction L (FIG. 6D) of traces 1509A-1509M, so that atleast a portion of each beam is reflected from the exposed surface 1505(FIG. 6B) of the structure. In such a case, the electrical signalobtained from two or more reflected portions indicates a profile of theexposed surface 1505. An optional polarizing beam splitter can be usedto limit the measurement to just the two portions that are reflected bytraces 1509A-1509M (as opposed to other portions that may be reflected,e.g. by the underneath surface 1508S.

Note that for convenience, FIGS. 7A-7E are labeled with many of the samereference numerals as FIGS. 6A-6E. In one embodiment, a polarizer 1523(FIG. 7E) is interposed between Wollaston prism 1520 and lens 1521 andis oriented at 45° relative to the two orthogonal polarizationdirections of beams 1502 as and 1502 ap generated by prism 1520, so thatthese beams have polarization directions parallel to one another whenincident on lens 1521. For example, as illustrated in FIGS. 7A and 7C,both beams 1502 as and 1502 ap can be oriented perpendicular to traces1504A-1504M so that the reflected signal provides a measure of theprofile of surface 1503S located underneath traces 1504A-1504M.Alternatively, as illustrated in FIGS. 7B and 7D, both beams 1502 bs and1502 bp can be oriented parallel to traces 1509A-1509M, so that thereflected signal provides a measure of the profile of surface 1509Lwhich is formed by traces 1509A-1509M and layer 1508 interdigitatingtherebetween.

In one specific implementation, the following three techniques are usedto perform a number of measurements. In a first technique called “MetalIllumination” (MI), one laser heats a metal film under linear responseconditions so that the film's temperature is modulated at a frequencywhich is selected to be sufficiently small to cause a majority of theheat to transfer by diffusion. The peak temperature is under the beamfocal spot, and is a function of the thermal conductivity andcross-section (for a line) or thickness (for a film). A second lasermeasures the reflectance, which is a function of the surfacetemperature. If the conductivity is well controlled, the MI measurementcorrelates to the line cross-section or film thickness. It is typicallyapplicable to films>300 Å thick. The heating beam is normally the 830 nmlaser, which may be linearly or circularly polarized.

A second technique called “Polarized Infrared DC Reflectance” (PIR)measures the reflectance of a polarized, normal incidence laser beam.Apparatus 125 (FIG. 1D) has lasers of two wavelengths available: 830 and980 nm. The 830 nm beam is circularly polarized and the 980 nm beam isunpolarized. Polarization directions are selected using the polarizingbeam splitter in the detector, or through the use of a polarizer flippedinto the beam to select a polarization from the circularly polarizedbeam.

A third technique called “Interferometric Surface Profiling” (SP)measures the phase shift between two closely spaced beams withorthogonal or like polarization. This provides a measure of the heightdifference of the surface at the focus, thereby measuring the localslope. Integrating the slope over the length of a scan comprised ofmeasurements at a number of sites provides the surface profile.

The measurements are in general applied to damascene structures,although they may be used with conventional metal layers as well. Thevarious defects of a structure that can be identified by themeasurements described herein include (see FIG. 8) doming, erosion,oxide thickness, trace cross-section, trace thickness, groove depth, padthickness, dishing, barrier thickness, etc which are described below indetail.

Doming (via PIR): The pattern dependent thickness variation of aninsulator layer left after polishing of the insulator layer, appearingas a domed layer above the array.

Erosion (via SP or MI): The pattern dependent line thickness variationin the fine-line array area, appearing as a depressed region in themiddle of the array.

Oxide thickness (via PIR): The thickness of an insulator layer asmeasured from the surface of the underlying layer to the surface of theinsulator.

Line cross-section (via MI): The cross-section area of an individualtrace (height×width).

Line height (via PIR & MI): The distance from the top to bottom of anindividual trace.

Groove depth (via PIR): The depth of the groove in the insulating layer.The conductive trace is formed within the groove.

Dishing (via MI or SP): The depression in the surface of a pad regionafter polishing.

Pad thickness (via MI): The thickness of the conductive layer in the padregion.

Barrier thickness (via MI): The thickness of the thin layer formedbetween the pad and the insulator. The barrier also exists between thelines and the insulator in the arrays.

Post electrodeposition topography (via SP): Before polishing, aconductive layer covers the entire surface. This layer is polished off,leaving the lines and pads. The surface has a topography that is patterndependent.

Thin layers (via PIR): A variety of thin layers may be used in theprocess. When thinner than e.g. 400 Å, these layers may be transparent.They are used for barriers, silicides, anti-reflection coatings, andother applications.

Resistance per unit length (line cross-section) (via MI): The MImeasurement is used to characterize the cross-section of fine patternedtraces. Assuming the conductivity of the material of the trace isconstant, the output is resistance per unit length, which variesinversely with the line cross-section. Because the cross-section is theproduct of the width and height of the trace, comparing the results ofthis measurement to a thickness measurement will give variation inwidth. A number of factors can cause width variation, including changesin critical dimensions (CDs), voiding, and barrier thickness. Themeasurement is also sensitive to adhesion, since it relies on thermalleakage into the surrounding insulator.

For performing the MI measurement, the heating and probing beams arepolarized along the length of the traces to eliminate sensitivity to theunderlying insulating layer. In this case, the measurement can beperformed at any level of metal as long as the pitch is less than thewavelength or the trace width is greater than the spot size (i.e. beamdiameter). One, embodiment has a sensitivity requirement ofcross-section variation<5%.

Numerous modifications and adaptations of the above-describedembodiments, implementations, and examples will become apparent to aperson skilled in the art of using lasers to measure properties ofsemiconductor wafers. For example, in an alternative embodiment, insteadof using a laser to generate heating beam 101, another heat source (suchas an electron gun and electron focusing column that forms an electronbeam) is used to modulate the temperature T. Also, a probe beam used tomeasure the average trace thickness as described herein can consist ofx-rays, in which case there is no need for the wavelength to be longerthan the pitch.

Note that the above-described method and apparatus can be used withtraces of any metal (such as copper or aluminum) or any silicide (suchas titanium, cobalt, or platinum), irrespective of whether or not thetraces have been annealed.

Moreover, instead of various shades of grey or color in a map, a contourmap may be displayed, with contour lines connecting dies havingapproximately the same measurements. For example, dies 2, 4, 7 and 8that fall within the range 0.47-0.48 illustrated in FIG. 3E may be shownconnected by a first contour line (or color), dies 1 and 3 maybe shownconnected by a second contour lie, dies 6 and 10 may be shown connectedby a third contour line, and so-on.

Note that a measure of thickness of a conductive layer as describedherein can be used to obtain a surface profile of the conductive layer,e.g., to identify a dimple that may be formed during polishing.

Another embodiment of a method and apparatus of the type describedherein illuminates a region of a wafer (having a number of traces overan underlying layer of the type illustrated in FIG. 1A) with polarizedwhite light, and measures the color of the reflected light, e.g. with acamera (and optionally an image processor). Films (such as layer 13 inFIG 1A) that are sufficiently thin (e.g. about 1-2 μm thick) have areflectance that is a function of wavelength, and therefore reflectlight of a color that depends on the thickness (e.g. thickness t in FIG.1A).

When the incident light is polarized so that the electric field isoriented across the traces, the polarization removes at least some ofthe effect of the overlying traces (e.g. traces 11A-11N in FIG. 1A)depending on the orientation. Therefore, when using light polarizedperpendicular to the longitudinal direction of the traces, only acolored region is seen whose color varies with thickness of theunderlying film. The following table indicates the change in color ofthe reflected light as a function of thickness of the underlying layer.

Film Thickness (μm) Color of reflected light 0.05 Tan 0.07 Brown 0.10Dark violet to red violet 0.12 Royal blue 0.15 Light blue to metallicblue 0.17 Metallic to very light yellow green 0.20 Light gold or yellowslightly metallic 0.22 Gold with slight yellow orange 0.25 Orange tomelon 0.27 Red violet 0.30 Blue to violet blue 0.31 Blue 0.32 Blue toblue green 0.34 Light green 0.35 Green to yellow green 0.36 Yellow green0.37 Green yellow 0.39 Yellow 0.41 Light orange 0.42 Carnation pink 0.44Violet red 0.46 Red violet 0.47 Violet 0.48 Blue violet 0.49 Blue 0.50Blue green 0.52 Green (broad) 0.54 Yellow green 0.56 Green yellow 0.57Yellow to “yellowish” (not yellow but is in the position where yellow isto be expected. At times it appears to be light creamy gray or metallic)0.58 Light orange or yellow to pink borderline 0.60 Carnation pink 0.63Violet red 0.68 “Bluish” (Not blue but borderline between violet andblue green. It appears more like a mixture between violet red and bluegreen and looks grayish) 0.72 Blue green to green (quite broad) 0.77“yellowish” 0.80 Orange (rather broad for orange 0.82 Salmon 0.85 Dull,light red violet 0.86 Violet 0.87 Blue violet 0.89 Blue 0.92 Blue green0.95 Dull yellow green 0.97 Yellow to “yellowish” 0.99 Orange 1.00Carnation pink 1.02 Violet red 1.05 Red violet 1.06 Violet 1.07 Blueviolet 1.10 Green 1.11 Yellow green 1.12 Green 1.18 Violet 1.19 Redviolet 1.21 Violet red 1.24 Carnation pink to salmon 1.25 Orange 1.28“Yellowish” 1.33 Sky blue to green blue 1.40 Orange 1.45 Violet 1.46Blue violet 1.50 Blue 1.54 Dull yellow green

A method that measures the color of reflected light may be performed asfollows. A beam of white light generated by a white light source(such asa halogen lamp) 826 (FIG. 2G) is polarized by a polarizer 804 a toobtain a beam of polarized white light. Apparatus 800C illustrated inFIG. 2G has many of the same components as apparatus 800A of FIG. 2D.Therefore, many of the reference numerals in FIG. 2G are same as thereference numerals in FIG. 2D, to denote the same components. Notehowever, that instead of a source of monochromatic light 801 inapparatus 800A, a source of white light 826 is used in apparatus 800C.

The polarized white light from polarizer 804 a is used to illuminate aregion of structure 807 with the polarization perpendicular to traces onstructure 807. Thereafter, a color of a portion of light reflected fromstructure 807 is measured, e.g. by use of an eyepiece lens 827, a camera828, and optionally a vision system 829.

Once the color is measured (either by human observation or by an opticalinstrument), the above table is used with the measured color to look upthe thickness t. In a variant of the method, instead of looking upthickness (which is an absolute value), a relative difference inthickness is measured (either qualitatively or quantitatively) bycomparing the colors obtained from two (or more) different regions ofstructure 807, thereby to obtain a corresponding change in thickness ofthe layer underlying the traces.

In one implementation, one or more measurements of the type describedherein are made by a circuit 600 (FIG. 9) that uses a photodiode (e.g.either of diodes D1 and D2 to generate a current (e.g. 1-2 milli amps)in response to the intensity of light incident on the photodiode.Thereafter, an amplifier U4 (FIG. 9) converts the current from thephotodiode into a voltage (e.g. 2-4 volts). Amplifier U4 is coupled to afilter U10 that filters out high frequency noise (e.g. from power lines;e.g. U10 may suppress any signal outside the frequency range 100 Hz to 5KHz).

Thereafter, an amplifier U11 amplifies the varying component (alsocalled “ac” component) of a measured signal by a gain that is selectableby the user (e.g. the gain may be any one of 1, 2, 4, 8, 16, 32, 64 and128). The gain may be selected by the user depending on the structure807 (FIG. 8) that is currently under examination, and the type ofsignals being obtained from the measurement. If necessary, an optional10×gain amplifier may be used to further amplify the measured signal.The resulting signal is provided to a lock-in amplifier for processingas described herein.

In another implementation, a signal from another photodiode D2 isamplified (as described above, but by amplifier U7). In addition tosumming the measured signals, these signals can be compared to oneanother, e.g. by an amplifier U1 which provides a difference signal. Thedifference signal is proportional to a property of the wafer, such assurface roughness.

In an alternative implementation, signals from each of amplifiers U4 andU7 are supplied to a summer (not shown) that in turn provides to filterU10 a signal that is the sum of the two signals obtained from the twophotodiodes D1 and D2, for use as described herein.

Numerous modifications and adaptations of the above-describedembodiments, implementations, and examples are encompassed by theattached claims.

1. A method for evaluating a structure, the method comprising:illuminating a region of the structure with a beam, the structure havinga plurality of lines passing through said region; generating anelectrical signal indicative of an attribute of a portion of the beam,the portion being reflected from said region; repeating the acts of“illuminating” and “generating” in another region of the structurehaving another plurality of lines, thereby to obtain another electricalsignal; and comparing said electrical signal with said anotherelectrical signal to identify variation of a property in said regionrelative to said another region.
 2. The method of claim 1 wherein: theattribute being measured is color.
 3. The method of claim 1 wherein: thebeam is nonpolarized; the attribute is a first intensity of a firstcomponent of said portion polarized in a direction substantiallyperpendicular to one of the lines, and the electrical signal ishereinafter “first electrical signal”; and the method further comprisesgenerating a second electrical signal indicative of a second intensityof a second component of said portion polarized in a directionsubstantially parallel to said one of the lines.
 4. A method forevaluating a structure, the method comprising: illuminating a region ofthe structure with a beam, the structure having a plurality of linespassing through said region; generating an electrical signal indicativeof an attribute of a portion of the beam, the portion being reflectedfrom said region; repeating the acts of “illuminating” and “generating”in another region having another plurality of lines, thereby to obtainanother electrical signal; and comparing said electrical signal withsaid another electrical signal to identify variation of a propertybetween said region and said another region; wherein the attribute beingmeasured is color.
 5. The method of claim 4 wherein: said region is at apredetermined location in the structure; and said another region is atsaid predetermined location in another structure.
 6. The method of claim4 wherein: said region is at a first location in the structure; and saidanother region is at a second location in said structure.
 7. A methodfor evaluating a structure having at least a plurality of lines and alayer in contact with said lines, the method comprising: illuminating aregion of the structure using a beam of electromagnetic radiation, thestructure having a plurality of lines in said region, the beam having awavelength greater than or equal to a pitch between at least two linesin the plurality, said two lines being each at least substantiallyparallel to and adjacent to the other; end generating an electricalsignal indicative of color of a portion of the beam, the portion beingreflected from said region.
 8. The method of claim 7 wherein: the linesare conductive; the structure is a wafer having formed therein aplurality of integrated circuit dice; and the method further compriseschanging a process parameter used in creation of another wafer based onthe electrical signal.
 9. The method of claim 7 further comprising:repeating the acts of“iilurninating”and “generating”in another regionhaving another plurality of lines, thereby to obtain additionalelectrical signal for said another region; and comparing said electricalsignal with said another electrical signal to identify variation of amaterial property between said region and said another region.
 10. Themethod of claim 7wherein: the attribute is a first color of a firstcomponent of said portion polarized in a direction perpendicular to oneof the two lines, and the electrical signal is hereinafter “firstelectrical signal”; and the method further comprises generating a secondelectrical signal indicative of a second color of a second component ofsaid portion polarized in a direction parallel to said one of the twolines.
 11. The method of claim 10 wherein: the acts of generating areperformed contemporaneously.
 12. The method of claim 11 wherein: thelines are conductive; the structure is a wafer having formed therein aplurality of integrated circuit dice; and the method further compriseschanging a process parameter used in creation of another wafer if thefirst electrical signal differs from the second electrical signal by apredetermined limit.
 13. A method for evaluating a structure having atleast a plurality of lines and a layer in contact with said lines, atleast two lines in the plurality being each at least substantiallyparallel to the other, the method comprising: illuminating the structurewith a beam of electromagnetic radiation having at least two polarizedcomponents wherein a first component is substantially parallel to thetwo lines, and a second component is substantially perpendicular to thetwo lines; generating a first electrical signal indicative of color of aportion of the first component reflected by at least said two lines; andgenerating a second electrical signal indicative of color of a portionof the second component reflected by the layer; wherein the acts ofgenerating are performed at least contemporaneously relative to oneanother.
 14. The method of claim 13 wherein: the acts of generating areperformed simultaneously relative to one another.
 15. A method forevaluating a structure having at least a plurality of lines and a layerin contact with said lines, at least two lines in the plurality beingeach at least substantially parallel to the other, the methodcomprising: illuminating a first region of the structure with a firstbeam of electromagnetic radiation; illuminating a second region of thestructure with a second beam of electromagnetic radiation; generating afirst electrical signal indicative of color of a portion of the firstbeam reflected from the first region; generating a second electricalsignal indicative of color of a portion of the second beam reflectedfrom the second region; and using a difference between the firstelectrical signal wit the second electrical signal as a profile of asurface in the structure.
 16. The method of claim 15 wherein: each ofthe first beam and the second beam is polarized; and the first beam hasa polarization direction perpendicular to the polarization direction ofthe second beam.
 17. The method of claim 15 wherein: each of the firstbeam and the second beam is polarized; and the first beam has apolarization direction parallel to the polarization direction of thesecond beam.
 18. The method of claim 15 wherein: the lines areconductive; the structure is a wafer having formed therein a pluralityof integrated circuit dice; and the method further comprises changing aprocess parameter used iii creation of another wafer if the firstelectrical signal differs from the second electrical signal by apredetermined limit.
 19. An apparatus for evaluating a wafer having aplurality of lines, said apparatus comprising: a source of a beam ofelectromagnetic radiation having a wavelength greater than or equal to apitch between at least two lines in said wafer, said two lines beingeach at least substantially parallel to and adjacent to the other; acamera that measures color located in a path of a portion of the bean,the portion being reflected from said region; a memory; and a comparatorcoupled to each of said memory and said camera.
 20. The apparatus ofclaim 19 further comprising: a polarizer located in a path of said beambetween said source and said wafer.
 21. The apparatus of claim 20further comprising: a stage supporting said wafer, the stage holding thewafer to orient the lines in a predetennined direction; wherein thepolarizer is aligned to orient polarization of the beam in saidpredetermined direction.
 22. The apparatus of claim 20 furthercomprising: a stage supporting said wafer, the stage holding the waferto orient the lines in a predetermined direction; wherein the polarizeris aligned to orient polarization of the beam perpendicular to saidpredetermined direction.
 23. The apparatus of claim 19 wherein: saidmemory is coupled to the camera; and during operation, said memorystores a first signal generated by the camera at a first time, the firstsignal being related to a first region in the wafer and said comparatorcompares the first signal with a second signal generated by the cameraat a second tine, the second signal being related to a second region.24. An apparatus for evaluating a wafer having a plurality of lines,said apparatus comprising: a source of abeam of electromagneticradiation having a wavelength greater than or equal to a pitch betweenat least twa lines in said wafer, said two lines being each at leastsubstantially parallel to and adjacent to the other; a polarizing beamsplitter located in a path of said beam between said source and saidwafer; and a polarizer located in a path of said beam between thepolarizing beam splitter and the wafer.
 25. The apparatus of claim 24wherein: the polarizing beam splitter is a Wollaston prism.
 26. A methodfor evaluating a structure having at least a plurality of lines and alayer in contact with said lines, at least two lines in the pluralitybeing each at least substantially parallel to the other, the methodcomprising: polarizing a beam of white light to obtain a beam ofpolarized white light; illuminating a region of the structure with saidbeam of polarized white light; and measuring a color of a portion ofsaid beam of polarized white light reflected from the region.
 27. Themethod of claim 26 wherein: the beam has a wavelength greater than adistance between center lines of the two lines.
 28. The method of claim27 wherein: the beam is polarized in a direction substantially parallelto one of the two lines; and said portion is reflected by the two lines.29. The method of claim 28wherein: the structure includes a layerlocated between a source of the beam and the two lines; and the layer isat least partially transmissive, so tat said portion passes through thelayer.
 30. The method of claim 28 wherein said portion passes trough atrausmissive medium other than a layer of a semiconductor wafer, thetransmissive medium being located between a source of the beam and thetwo lines.
 31. The method of claim 28 wherein the beam is hereinafter“first beam ”, the method further comprising: illuminating said regionwith a second bean, a portion of energy in said second beam that is notreflected by said region being converted into heat, said second beamhaving an intensity modulated at a predetermined frequency beingsufficiently small to cause a majority of said heat to transfer bydiffusion from said region; wherein the portion of the first beam sensedin the act of generating is modulated in phase with modulation of saidsecond beam.
 32. The method of claim 31 wherein: the second beam ispolarized in a direction parallel to one of the two lines; and at leasta portion of the second beam reflects from a surface of said “one of thetwo lines”.
 33. The method of claim 27 wherein: the beam polarized in adirection at least substantially perpendicular to one of the two lines;the structure includes a layer located between the semiconductorsubstrate and the two traces; and said portion is reflected by thelayer.
 34. The method of claim 33 wherein: the lines are embedded withinthe layer so that at least a part of the layer is located between thetwo lines and at least some of said portion passes through said part.35. The method of claim 33 wherein the beam contains photons havingenergy equal to or lower than bandgap energy of a semiconductor materialin said region, the method further comprising: creating a plurality ofcharge carriers in the layer, the charge carriers being modulated at afrequency that is sufficiently low to avoid creation of a wave of thecharge carriers; wherein the portion of the beam used in the act ofgenerating is modulated at said frequency and in phase with modulationof the charge corners.
 36. The method of claim 26 wherein the structureis a semiconductor wafer, the method further comprising, prior to theacts of illuminating and “generating”: adding dopant atoms to at leastsaid region; and creating said plurality of lines in at least saidregion. the beam has a wavelength greater than a distance between centerlines of the two lines.
 37. The method of claim 36 further comprising:changing a process parameter used in either one of the acts of addingand creating, if the variation is greater than a predetermined limit.38. The method of claim 26 wherein said region is a first region andsaid color is a first color, the method further comprising: illuminatinga second region of the structure with said beam of polarized whitelight; measuring a second color of a portion of said beam of polarizedwhite light reflected from the second region; and using a difference insaid first color and said second color to indicate a correspondingchange in thickness of said layer.
 39. The method of claim 26 whereinsaid measuring is performed by an optical instrument.